One "mysterious structure" has been built at the main square in North Korea's capital city of Pyongyang, which appears to be related to its party congress slated for next month, a US website monitoring the communist country has said.

According to 38 North on Friday, a small building surrounded by a high wall has been built in Kim Il-sung Square "right across the street from the viewing platform used by (leader) Kim Jong-un and party officials during parades."

It also said that another one was set up adjacent to the structure but outside of the walled-off area, citing recent satellite imagery, adding that the construction appears to have begun last week.

"The purpose of the buildings is unclear, although they could be related to the upcoming Eighth Party Congress that is expected to take place in January," 38 North said.

North Korea has prepared for a key session of its ruling Workers' Party in January, at which it could unveil a new national strategy to deal with a series of challenges, such as the COVID-19 pandemic and prolonged global sanctions.

It also plans to stage a parade to mark the event, according to Seoul's national intelligence agency. The square has been a venue for such massive events, including the latest one held in October.

"At the Mirim Parade Training Ground in east Pyongyang, where parade rehearsals often take place, several formations of marching troops and a number of cargo trucks and tracked vehicles are present" according to the website. "This activity is consistent with parade preparations, although at a much smaller scale than what took place ahead of the October 10 military parade." (Yonhap)